1. Field of the Invention
The present invention relates to an electrode material, an electrode and a lithium ion battery, and particularly to an electrode material that is preferably used as a positive electrode material used in batteries and, furthermore, a positive electrode material used in lithium ion batteries, an electrode containing the above-described electrode material, and a lithium ion battery including a positive electrode made of the above-described electrode.
2. Description of Related Art
In recent years, as a battery expected to attain a decrease in size and weight and an increase in capacitance, non-aqueous electrolytic solution secondary batteries such as lithium ion batteries have been proposed, and have been put into practical use.
The lithium ion battery is made up of a positive electrode and a negative electrode which can reversibly accept and expel lithium ions and a non-aqueous electrolyte.
Regarding negative electrode materials of lithium ion batteries, generally, a carbon-based material or a Li-containing metallic oxide which can reversibly accept and expel lithium ions, such as lithium titanate (Li4Ti5O12), is used as a negative electrode active material.
On the other hand, regarding positive electrode materials of lithium ion batteries, a Li-containing metallic oxide which can reversibly accept and expel lithium ions, such as lithium iron phosphate (LiFePO4) or an electrode material mixture containing a binder and the like is used as a positive electrode active material. In addition, a positive electrode of a lithium ion battery is formed by applying the electrode material mixture to a surface of a metal foil called a collector.
The lithium ion battery has a light weight, a small size and a high energy compared to secondary batteries of the related art, such as lead batteries, nickel-cadmium batteries and nickel-hydrogen batteries, and therefore the lithium ion battery is used not only as a small-scale power supply for portable electronic devices such as mobile phones and notebook computers but also as a large-scale stationary emergency power supply.
In addition, in recent years, studies are being conducted regarding the use of the lithium ion battery as a high-output power supply in plug-in hybrid vehicles, hybrid vehicles, electromotive tools and the like, and batteries being used as a high-output power supply in the above-described machines are required to have high-speed charge and discharge characteristics.
However, there is a problem in that an electrode active material of the related art, for example, an electrode material containing a lithium phosphate compound which can reversibly accept and expel lithium ions has low electron conductivity. Therefore, in order to increase the electron conductivity of electrode materials, an electrode material is proposed in which the surfaces of particles of an electrode active material are coated with an organic compound which serves as a carbon source, and then the organic compound is carbonized, whereby a carbonaceous coat is formed on the surface of the electrode active material, and carbon in the carbonaceous coat is interposed as an electron-conducting material (Patent Documents 1 and 2 and the like).
Meanwhile, as an electrode active material that realizes stable charge and discharge cycle characteristics, electrode active materials formed by joining the particles of a carbonaceous coated electrode active material having a carbonaceous coat formed on the surface together through an electron conductive substance such as carbon for which the N-methyl-2-pyrrolidone (NMP) oil absorption amount is controlled to be in a range of 25 g/100 g to 35 g/100 g (Patent Document 3) or for which the N-methyl-2-pyrrolidone (NMP) oil absorption amount is controlled in a range of 25 g/100 g to 200 g/100 g (Patent Document 4) have been proposed.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2001-15111
[Patent Document 2] Japanese Laid-open Patent Publication No. 2012-133888
[Patent Document 3] Japanese Laid-open Patent Publication No. 2012-104290
[Patent Document 4] PCT Japanese Patent Domestic Re-publication No. 2011-033707